Arsenic is a well-documented human carcinogen. Our goal is to address the central hypothesis that phosphorylation of histones and their upstream kinases play an important functional role in arsenic-induced cell transformation and carcinogenesis. Specific Aim 1 is to study the role of histone phosphorylation in arsenic-induced cell transformation; Specific Aim 2 is to investigate and identify the histone kinases that phosphorylate histone H3 and H2B at different amino acid residues; Specific Aim 3 is to study the crystal structure of histone kinase RSK2, perform in- silico screening and design RSK2 inhibitors for suppressing arsenic-induced histone phosphorylation and cell transformation; and Specific Aim 4 is to study the role of RSK2 in arsenic/ultraviolet A (UVA)-induced skin carcinogenesis and determine RSK2's potential as a target for chemoprevention of cancer. The strategy for Specific Aim 1 is to use point mutations at key phosphorylation sites of histone H3 and H2B siRNA gene knockdown and overexpressing stable cell lines to test the role of H3 and H2B in soft agar cell transformation assays. For Specific Aim 2, we will use in vitro kinase assays, specific mutations, LTQ Orbitrap hybrid mass spectrometer analysis and RSK2 knockout cells as well as inhibitors of RSK2. In Specific Aim 3, we will use x-ray crystallography to determine the structure of RSK2. Then we will use a super computer to screen a database of 2.5 million chemicals to find inhibitors for RSK2 to be tested in an in vitro kinase assay. In Specific Aim 4, we will test the effect of the RSK2 inhibitor kaempferol and RSK2 knockout mice in UVA/arsenic-induced mouse skin carcinogenesis. Such knowledge will facilitate the design of more effective and specific strategies with fewer side effects for chemoprevention of arsenic- induced cancer.